Hess's Law

A scientific law is a simplified, general summary of what we can readily observe in God's world -- His ordinary way of sustaining and governing the creation. Hess observed that changes in state properties like enthalpy can be determined either directly or by adding together a series of steps in an overall change in any order. In your experiment, you switched the order of the three reactants to test this observation. Was the overall deltaH per gram of NaOH about the same regardless of the order in which you mixed reactants? Hopefully! Hess's observation and the idea of state property are two somewhat redundant concepts. We can observe what Hess did BECAUSE of state properties. Any property that can be calculated simply as a change between final (products) and initial (reactants) states is a state property. H, T, P, V... are all state properties. q and w are path properties. Heat and Work are the pathways of energy transfer so we don't talk about delta q or delta w. Either energy is transferred in an organized way or a disorganized way. There is no meaning to a change in the transfer of energy -- transfer IS the change. Hess's Law allows us to cleverly calculate properties such as deltaH of a reaction (heat of reaction) using heats of formation values. It is "clever" since we do not have to carry out a direct reaction, instead we relate experimental values indirectly to get what we want. We end up with a more general and accurate method for heat of reaction than by worrying about gas-phase average Bond Enthalpies. We'll use Hess's Law as a tool to determine values for other state property changes like entropy (S) and Gibbs free energy (G) coming up...